New Hydrogen Peroxide Adducts of Alkali Metal Tetracyanoplatinates
A2 [Pt(CN)4 ] · H2 O2 (A = K, Rb, Cs)
Claus Mühle, Eva-Maria Peters, and Martin Jansen
Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
Reprint requests to Prof. Dr. Martin Jansen. Fax: ++49-0711-6891502. E-mail: [email protected]
Z. Naturforsch. 2009, 64b, 111 – 115; received October 13, 2008
Dedicated to Professor Otto J. Scherer on the occasion of his 75 th birthday
The title compounds have been synthesized by adding hydrogen peroxide to an aqueous solution of A2 [Pt(CN)4 ] (A = K, Rb, Cs). They grow as yellow needles after concentrating and cooling
to 4 ◦C. The structures were elucidated from single crystal analysis. The isostructural compounds
crystallize monoclinically, in space group C2/c with Z = 4. K2 Pt(CN)4 · H2 O2 : a = 13.3751(7), b =
11.2713(6), c = 6.5461(3) Å, β = 105.432(1)◦ , V = 951.3(3) Å3 . Rb2 Pt(CN)4 · H2 O2 : a = 13.6103(2),
b = 11.6759(1), c = 6.5683(7) Å, β = 106.588(2)◦ , V = 1000.3(2) Å3 . Cs2 Pt(CN)4 · H2 O2 : a =
13.9569(2), b = 12.2023(2), c = 6.5857(9) Å, β = 107.590(3)◦ , V = 1069.1(2) Å3 . As a remarkable feature, the hydrogen bonds O–H···N vary significantly with the cation size: in the Cs compound
the O–H bonds are weakest, and the N· · · H interactions are strongest. All three compounds were
characterized by differential thermal analysis, thermogravimetry and infrared spectroscopy.
Key words: Crystal Structure, Alkali Metal Tetracyanoplatinate, Hydrogen Peroxide,
Infrared Spectroscopy
Introduction
Substantial progress in the cyanoplatinate chemistry has been marked by the structural characterization of partially oxidized salts by Krogmann in
the 1960ies [1]. He revealed a significant contraction of the Pt–Pt distances in the oxidized products
K2 [Pt(CN)4 ]X0.33 · y H2 O (X = Cl, Br) to distances
as short as 2.88 Å [1], which is only slightly longer
than in metallic platinum (2.77 Å), suggesting significant bonding interactions between the partially filled
Pt dz2 orbitals. The oxidized cyanoplatinates thus can
be considered as one-dimensional metals, in accordance with their relatively high electron conductivity. The first experiments to obtain partially oxidized
potassium cyanoplatinates by adding hydrogen peroxide and sulfuric acid to respective aqueous solutions were performed by Levy [2] at the beginning
of the 20th century. In 1968, Krogmann [3] solved
the structure and determined the composition of the
product to be K1.74 [Pt(CN)4 ] · 1.8 H2 O, exhibiting a
Pt–Pt distance of 2.96 Å. Since Krogmann’s pioneering work the amazing structure-property correlations in this class of compounds have been treated
in a huge number of publications. A comprehen-
sive review has been given by Williams in [4], including reports on Rb1.6 [Pt(CN)4 ] · 2 H2 O [5] and
Cs1.75 [Pt(CN)4 ] · y H2 O [4]. We have continued in
this field [6, 7] focusing on getting more information
about the partially oxidized cyanoplatinates, in particular how they form. Our attempts to synthesize the
rubidium-deficient cyanoplatinate following the conventional route suggested by Levy [2] and Williams
[5] sometimes resulted in the formation of yellow needles of an unknown phase. We found out that hydrogen peroxide in such instances did not oxidize platinum at neutral conditions, instead yellow needles
form which are hydrogen peroxide adducts of the formula A2 [Pt(CN)4 ] · H2 O2 (A = K, Rb, Cs). Here we
report on their structure analyses by X-ray crystallography, and characterization of the new adducts by infrared spectroscopy (IR), thermogravimetry (TG) and
differential thermal analysis (DTA).
Experimental Section
Synthesis
K2 [Pt(CN)4 ] · 3 H2 O was obtained from K2 [PtCl4 ]
(Chempur, 99.9 %) and KCN as described previously [6, 8].
This salt was recrystallized three times from water to re-
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C. Mühle et al. · Hydrogen Peroxide Adducts of Alkali Metal Tetracyanoplatinates A2 [Pt(CN)4 ] · H2 O2
Formula
Formula weight, g mol−1
Crystal size, mm3
Crystal system
Space group
Z (formula units)
a, Å
b, Å
c, Å
β , deg
V , Å3
T, K
ρcalc , g cm−3
Radiation; λ , Å
µ (MoKα ), mm−1
θ range for data collection, deg
Reflections collected
Independendent reflections
Goodness of fit on F 2
Final R1 / wR2 [I ≥ 2σ (I)]
Final R1 / wR2 (all data)
∆ρfin (max / min), e Å−3
Deposition number CSD
K2 [Pt(CN)4 ] · H2 O2
411.39
0.40 × 0.15 × 0.15
monoclinic
C2/c (no. 15)
4
13.3751(7)
11.2713(6)
6.5461(3)
105.432(1)
951.3(3)
296(2)
2.87
MoKα , 0.71073
15.596
2.40 – 37.02
12789
2391
0.997
0.0131 / 0.0320
0.0225 / 0.0355
1.81 / −1.11
418913
Rb2 [Pt(CN)4 ] · H2 O2
504.13
0.30 × 0.15 × 0.05
monoclinic
C2/c (no. 15)
4
13.6103(2)
11.6759(1)
6.5683(7)
106.588(2)
1000.3(2)
296(2)
3.35
MoKα , 0.71073
23.668
2.34 – 37.35
12437
2532
0.959
0.0222 / 0.0472
0.02406 / 0.0529
1.19 / −2.06
418912
Table 2. Selected interatomic distances (Å) and angles (deg)
in A2 [Pt(CN)4 ] · H2 O2 (A = K, Rb, Cs).
Pt1 – Pt1
Pt1 – C1
Pt1 – C2
C1 – N1
C2 – N2
A1 – N1
A1 – N2
A1 – O1
N1 – H1
O1 – N1
O1 – N2
O1 – O1
O1 – H1
A=K
3.273(1)
1.984(2)
1.985(2)
1.145(2)
1.146(2)
2.838(9)
2.850(2)
2.840(6)
2.008(3)
2.825(1)
2.868(1)
1.451(3)
0.83(3)
A = Rb
3.284(1)
1.987(3)
1.988(3)
1.147(4)
1.144(4)
3.004(6)
3.075(8)
2.980(3)
2.083(1)
2.836(5)
2.942(7)
1.444(5)
0.79(4)
A = Cs
3.292(9)
1.995(7)
1.997(8)
1.149(9)
1.144(1)
3.209(9)
3.264(4)
3.180(4)
1.829(1)
2.800(9)
3.089(8)
1.419(1)
1.19(1)
C1 – Pt1 – C1
C1 – Pt1 – C2
C2 – Pt1 – C2
N1 – A1 – C2
N2 – A1 – C2
O1 – A1 – C2
O1 – A1 – N2
O1 – O1 – H1
180
90.2(1)
180
77.60(5)
160.56(5)
95.92(5)
103.54(5)
101.0(2)
180
91.3(1)
180
75.18(7)
161.90(7)
96.89(7)
105.17(8)
99.0(3)
180
92.0(3)
180
72.15(2)
161.8(2)
96.07(2)
106.6(2)
97.0(8)
move all traces of chloride. To a warm concentrated aqueous
solution of 200 mg K2 [Pt(CN)4 ] · 3 H2 O 0.01 mL of 35 %
H2 O2 was added. Yellow crystals of K2 [Pt(CN)4 ] · H2 O2
were formed after concentrating and subsequent cooling in
a refrigerator at 4 ◦C.
A concentrated aqueous solution of Ba[Pt(CN)4 ] · 4 H2 O
(Chempur, 99.9 %) was mixed with an excess of an aqueous solution of A2 SO4 (A = Rb, Cs) as described in [9]. After separating BaSO4 by filtrating, the clear residual solution
Cs2 [Pt(CN)4 ] · H2 O2
599.01
0.20 × 0.08 × 0.04
monoclinic
C2/c (no. 15)
4
13.9569(2)
12.2023(2)
6.5857(9)
107.590(3)
1069.1(2)
296(2)
3.72
MoKα , 0.71073
19.810
2.26 – 27.50
3977
1021
0.947
0.0305 / 0.0861
0.0389 / 0.0974
2.33 / −1.68
418911
Table. 1. Crystallographic data
of A2 [Pt(CN)4 ] · H2 O2 (A = K,
Rb, Cs).
was concentrated by heating, and 0.01 mL of 35 % H2 O2 was
added. Yellow needles of A2 [Pt(CN)4] · H2 O2 (A = Rb, Cs)
were obtained by cooling to 4 ◦C in a refrigerator.
Crystal structure determination
Single crystals of A2 [Pt(CN)4 ] · H2 O2 (A = K, Rb, Cs)
were selected under a microscope and sealed in Lindemann capillaries. All three compounds crystallize in space
group C2/c (no. 15). The intensity data were measured on
a Smart APEX CCD diffractometer (Bruker AXS) with a
graphite monochromator using ω scans. Absorption corrections were done by the program S ADABS [10]. The structures
were solved and refined using the S HELXTL program system [11]. The atomic positions of the platinum and the alkali
metals were found by Direct Methods, the positions of the
cyanide and peroxide groups could be located from Difference Fourier maps. The hydrogen atoms have also been localized, and refined applying isotropic displacement parameters. Refinement was done by full-matrix least-squares methods on F 2 . The crystallographic data are collected in Table 1,
selected atomic distances and angles are given in Table 2.
Further details of the crystal structure investigations
may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax:
(+49)7247-808-666; e-mail: [email protected],
www.fiz-karlsruhe.de/request for deposited data.html) on
quoting the deposition numbers given in Table 1.
Physical properties
Simultaneous DTA/TG/MS was performed with a
computer-controlled thermal analyzer (STA 409, Netzsch
C. Mühle et al. · Hydrogen Peroxide Adducts of Alkali Metal Tetracyanoplatinates A2 [Pt(CN)4 ] · H2 O2
113
GmbH, Germany). Samples of about 20 mg were placed in a
corundum crucible, heated to 900 ◦C with a rate of 10 ◦C/min,
and then cooled down to r. t. with the same rate. The whole
process was run under argon.
The infrared spectra were recorded on an IR Spectrometer (113v, Bruker, Germany). About 2 mg of the samples
were thoroughly mixed and ground with ∼400 mg of KBr
(Aldrich, 99+ %, dried at 100 ◦C), and subsequently pressed
into a pellet with a diameter of 10 mm. The frequencies of
the main absorption bands are given in Table 3, a spectrum
of K2 [Pt(CN)4 ] · H2 O2 is shown in Fig. 3.
Results and Discussion
Crystal Structures
Up to now, only one previous report on a hydrogen peroxide adduct of a platinum salt, [PtCl2 (NH3 )2 (OH)2 ] · 2 H2 O2 [12], is documented. The new hydrogen peroxide adducts A2 [Pt(CN)4 ] · H2 O2 (A = K,
Rb, Cs), presented here, crystallize monoclinically in
the space group C2/c, and the structures consist of
square planar tetracyanoplatinate groups stacked in a
staggered conformation along the c axis, thus forming one-dimensional chains of platinum atoms (Figs. 1
and 2). The same arrangement is observed in most hydrated and anhydrous cyanoplatinates [6, 7]. The platinum separations of 3.27 to 3.29 Å (Table 2) are in
Fig. 1. A perspective view of the crystal structure of
K2 [Pt(CN)4 ] · H2 O2 with square planar tetracyanoplatinate
groups. Black spheres represent platinum atoms, large grey
spheres – potassium atoms, small black spheres – carbon atoms, light-grey spheres – nitrogen atoms, small grey
spheres – oxygen atoms. The black lines mark the unit cell.
Fig. 2. A section of the crystal structure of
Cs2 [Pt(CN)4 ] · H2 O2 . Top: view onto the crystallographic
ab plane; bottom: view onto the ac plane. Black spheres
represent platinum atoms, large grey spheres – caesium
atoms, small black spheres – carbon atoms, light-grey
spheres – nitrogen atoms, small grey spheres – oxygen
atoms. The black lines mark the unit cell.
the range expected for non-oxidized cyanoplatinates,
but significantly shorter than in the monohydrates (c. f.
d(Pt–Pt) = 3.58 Å in K2 [Pt(CN)4 ] · H2 O [6], d(Pt–
Pt) = 3.42 Å in Rb2 [Pt(CN)4 ] · 1.5 H2 O [9] d(Pt–
Pt) = 3.54 Å in Cs2 [Pt(CN)4 ] · H2 O [13]), and pronouncedly larger than in the partially oxidized salt
K1.74 [Pt(CN)4 ] · 1.8 H2 O, d(Pt–Pt) = 2.96 Å [3] and in
A1.75 [Pt(CN)4 ] · y H2 O (A = Rb, Cs), d(Pt–Pt) = 2.88
to 2.94 Å [4], thus suggesting only weak direct interactions between the platinum atoms.
The alkali metal atoms are coordinated by six nitrogen atoms in the shape of a distorted trigonal prism
with two of the faces capped by oxygen atoms raising
the coordination number to eight. The distances d(A–
N1) ranging from 2.84 to 3.21 Å (Table 2) grow as is to
be expected from A = K to Cs, and are similar to those
known for other non-oxidized cyanoplatinates [6, 7].
114
C. Mühle et al. · Hydrogen Peroxide Adducts of Alkali Metal Tetracyanoplatinates A2 [Pt(CN)4 ] · H2 O2
Table 3. IR absorption bands (cm−1 ) of A2 [Pt(CN)4 ] · H2 O2
(A = K, Rb, Cs)a .
A=K
δ (Pt–C≡N)
410 s
δ (Pt–C)
503 s
ν (O–O)
870 w
δs/as (O–H)
1386 s
ν (C≡N)
2132 vs
νs (O–H)
2742 w
νas (O–H)
3229 s
a vs = very strong, s = strong, w = weak.
A = Rb
411 s
504 s
877 w
1398 s
2127 vs
2758 w
3231 s
A = Cs
411 s
505 s
878 w
1415 s
2120 vs
2780 s
3204 s
The symmetry-independent Pt1–C1, Pt1–C2 (1.984 –
1.997 Å) and C1–N1, C2–N2 bonds (1.144 – 1.149 Å)
of the planar cyanoplatinate groups show a small
spread over all compounds (Table 2). Along the c axis,
the adjacent tetracyanoplatinate groups are rotated
with torsion angles C–Pt–Pt–C different from 45◦ , decreasing from K (43.0◦) and Rb (39.6◦) to Cs (37.5◦).
This dihedral angle is regarded indicative of the degree
of the platinum 5dz2 orbital overlap [14]. Furthermore
this rotation of the Pt(CN)4 groups enables hydrogen
peroxide to form hydrogen bonds to the nitrogen atoms
with the characteristic distances of d(OH· · · N) = 2.8 to
3.1 Å and d(O–O) = 1.42 to 1.45 Å, similar to those
observed for [PtCl2 (NH3 )2 (OH)2 ] · 2 H2 O2 [12].
Remarkably, in Cs2 [Pt(CN)4 ] · H2 O2 the O–H
bonds with 1.19 Å are significantly longer and weaker,
than in the potassium and rubidium compounds, and
the hydrogen atoms appear to be shifted towards the
nitrogen atoms forming stronger N···H interactions.
The O1–O1 distances and the O1–N1 separations in
the cesium cyanoplatinate peroxide adduct clearly are
shorter than in the other two compounds but do not
induce a significant effect on the principal structural
features.
Depending on the cation size, the unit cell varies
only in the a and b axis but not significantly with respect to the c axis. Thus, only the hydrogen bonds
were affected by the type of the alkali cation but not
the Pt–Pt interactions of the platinum chains along the
c axis.
Physical properties
The decomposition of K2 [Pt(CN)4 ] · H2 O2 , as measured by DTA and TG-MS, starts at 100 ◦C with loss
of hydrogen peroxide. According to the MS the evolving entities are HO, H2 O and H2 O2 . At temperatures
higher than 700 ◦C the CN groups are also leaving.
Powder X-ray diffraction measurements of the solid
Fig. 3. Infrared spectrum of K2 [Pt(CN)4 ] · H2 O2 .
residues confirm a decomposition to elemental platinum. The thermal decompositions of Rb2 [Pt(CN)4 ] ·
H2 O2 and Cs2 [Pt(CN)4 ] · H2 O2 proceed in a similar
way.
In all three infrared spectra (one is shown
in Fig. 3, frequencies are given in Table 3) of
A2 [Pt(CN)4 ] · H2 O2 (A = K, Rb, Cs) the strong absorption bands at 2120 – 2132 cm−1 , which are associated with the valence vibrations of the cyanide groups
[15], and correspond to the ν (C≡N) valence vibrations
of anhydrous and hydrated alkali metal cyanoplatinates [6, 7], are similar in accord with the similar
Pt–Pt distances in all three compounds. The frequencies of the vibration of coordinated C≡N groups in
the tetracyanoplatinates are much higher than in KCN
(2080 cm−1 ) [6]. The strong bands at 503 – 505 cm−1
are assigned to the deformation vibrations of the Pt–C
bonds [15, 16]. The absorption bands of the deformation vibrations of the Pt–C≡N groups occur at 410 –
411 cm−1 [6, 14]. The hydrogen peroxide can be identified by the weak ν (O–O) valence vibration at 870
to 878 cm−1 [17], the δ (O–H) deformation and the
characteristic broad δ (O–H) valence vibrations. In the
K and Rb compounds, the deformation vibration occurs at similar frequencies at δs/as (O–H) = 1386 and
1398 cm−1 whereas in the Cs compound it is shifted to
1415 cm−1 . The symmetric valence vibrations νs (O–
H) at 2742 (K), 2758 (Rb) or 2780 cm−1 (Cs) relate to
the overtone of the deformation vibration with lower
intensity and at lower frequency. The asymmetric valence vibrations νas (O–H) at 3229 (K), 3231 (Rb) and
3204 (Cs) occur at lower frequencies for hydrogen peroxide than for water molecules [18] and reflect the
C. Mühle et al. · Hydrogen Peroxide Adducts of Alkali Metal Tetracyanoplatinates A2 [Pt(CN)4 ] · H2 O2
longer and weaker O–H bonds in Cs2 [Pt(CN)4 ] · H2 O2 ,
as compared to the K and Rb analogs. Thus, the findings on the hydrogen bonding distances varying from
the potassium and rubidium platinate to the cesium
representative seem to be corroborated by infrared
spectroscopy.
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Acknowledgements
The authors would like to thank Dr. Jürgen Nuss for the
single crystal data collection, Wolfgang König for the IR and
Dr. Christian Oberndorfer for the DTA/TG measurements.
The continuous financial support by the Fonds der Chemischen Industrie, and by the Max Planck Society is gratefully
acknowledged.
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